1. Field of the Invention
The present invention relates to a lens such as an inner-focus-type lens system, and more particularly to a control method for the position of the lens.
2. Description of Related Art
FIG. 11 shows the arrangement of an inner-focus-type lens system. Referring to FIG. 11, the inner-focus-type lens system is composed of a first lens group 1101 which is fixed, a second lens group 1102 for variation of magnification (hereinafter referred to as a variator lens), an iris 1103, a third lens group 1104 which is fixed, a fourth lens group 1105 which has both a focus adjusting function and a so-called compensation function of compensating for the shift of a focal plane due to the variation of magnification (hereinafter referred to as a focusing lens), and an image pickup surface 1106. As is well known, in the lens system arranged as shown in FIG. 11, since the focusing lens 1105 has both the focus adjusting function and the compensating function, the position of the focusing lens 1105 for focusing on the image pickup surface 1106 varies with object distances even if the focal length remains unchanged.
FIG. 12 is a diagram obtained by serially plotting positions of the focusing lens 1105 for focusing on the image pickup surface 1106 when the object distance is varied for various focal lengths. During zooming, one locus is selected from the loci shown in FIG. 12 according to the object distance, and if the focusing lens 1105 is moved along the selected locus, a zooming operation without blur can be attained. Accordingly, it is general that information on a plurality of loci shown in FIG. 12 is beforehand stored in a lens control microcomputer in some form, one locus is selected according to the positions of the focusing lens 1105 and the varaitor lens 1102, and a zooming operation is performed tracing the selected locus.
However, such a locus tracing method requires the value of each lens position counter to be reset to a predetermined value. In other words, if the value of the lens position counter deviates from a correct value, it becomes impossible to correctly select cam locus information which indicates the relationship between the variator lens position and the focusing lens position stored in the microcomputer, so that a correct cam locus can not be accurately traced during a zooming operation.
Therefore, it is widely practiced that, before an ordinary operation is performed after the power supply is turned on, the variator lens 1102 and the focusing lens 1105 are moved to the respective predetermined reset positions and each of the lens position counters is reset. In this case, the reset positions are generally to be set to the respective mounting positions of photodiodes incorporated in a lens barrel. Then, the lens positions where output signals of the respective photodiodes change are made to be the respective lens reset positions, and the values of the respective lens position counters are set respectively to values decided by balance adjustment of an optical system. Further, during the lens resetting operation, a picked-up image is made not to be outputted because of having large blur, and after completion of the lens resetting operation, a picked-up image is outputted.
In addition, in another proposed method, while a picked-up image is inhibited from being outputted even after completion of the resetting operation of the lens position counters, the lenses are returned to the respective positions where the lens were located before the turning-on of a power supply, so that an angle of view or the like is prevented from being varied due to the lens resetting operation.
Further, it is general in recent years that a pulse motor is used as the motor for driving a lens. In the pulse motor, since a rotation angle relative to a step pulse number is constant, the position of the lens can be detected by incrementing the step pulse number as it is, so that no encoder for position detection is required.
In the above-mentioned example, a lens resetting operation is performed only at a point of time when a power supply is turned on. Therefore, in the case of a surveillance camera or the like operating 24 hours, if an out-of-step phenomenon has occurred due to a change of environment or a disturbance, such as mechanical impact, shut-off of the power supply, electrical noise or radio noise, the power supply must be again turned on (restarting of the power supply) and the lens must be reset.
However, there is a problem in the case of a surveillance camera or the like that if the power supply is made to be turned off, thereby making it impossible to view a picked-up image, the significance of the surveillance camera would be lost.